Republican presidential candidate Mitt Romney on Thursday revealed a bit more about his tax history, telling reporters: "I never paid less than 13 percent" in the past 10 years.

The Obama campaign's response: "Prove it."

Romney's statement came during an appearance in South Carolina and followed weeks of demands — mostly from Democrats, but also from some Republicans — that Romney release several years of his tax returns.

He has released his 2010 tax returns, which showed an effective federal tax rate of 13.9 percent, and has promised to release his 2011 returns before the election.

The Senate's top Democrat, Majority Leader Harry Reid of Nevada, has claimed that an unnamed source told him Romney paid no taxes over the past decade.

"I did go back and look at my taxes and over the past 10 years I never paid less than 13 percent," said Romney, who was in South Carolina for a fundraiser. "I think the most recent year is 13.6 or something like that. So I paid taxes every single year. Harry Reid's charge is totally false."

Obama campaign spokeswoman Lis Smith responded in a statement: "Since there is substantial reason to doubt his claims, we have a simple message for him: prove it. ... Given Mitt Romney's secrecy about his returns ... he has forfeited the right to have us take him just at his word."

Romney is under no legal requirement to release more tax records. But his wealth (which Forbes estimates at $230 million), the campaign's emphasis on economic issues and tax rates, and recent political history would indicate that the calls for Romney to release more tax records will continue.

Since 1980 (when Republican Ronald Reagan released just one year of returns) only one party nominee or presumptive nominee other than Romney has released fewer than six years of returns, according to PolitiFact. That was Republican John McCain, who released two years of returns in 2008.

"The fascination with taxes I paid I find to be very small-minded compared to the broad issues that we face," Romney said Thursday.

In an interview with NBC conducted during the Olympics but scheduled to air later on Thursday, Romney's wife, Ann, also addressed the issue: "We have been very transparent to what's legally required of us," she told NBC. "There's going to be no more tax releases given."

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